Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
  • H04L1/0009—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the channel coding

Definitions

• the present invention generally relates to forward error correction (FEC).
• FEC forward error correction
• the error rate of wireless networks is not sufficient for video applications due to the possible large number of large packet losses/drops.
• Dropped or lost packets are sometimes not recovered, but in schemes where packet recovery is implemented, packets are recovered using retransmission or forward error correction (FEC), or a combination of both.
• FEC has been widely used to correct errors without requiring retransmission.
• FEC allows recovery of data contained in corrupted, dropped, or lost packets by transmitting redundant information, which can be used by the receiver to reconstruct missing data.
• FEC provides faster recovery of data than does retransmission and does not require a feedback channel.
• Static FEC techniques have been used by many researchers but fail to match FEC overhead with channel and network conditions.
• FEC frequency division multiple access
• the adaptation mechanism increases the number of redundant packets to send if the network condition is poor. If the network condition is satisfactory, then no redundant packets, or only a small number of redundant packets, need to be transmitted. Most of the prior art approaches are directed to bit-level FEC.
• One application for the adaptive FEC of the present invention is for video streaming over wireless networks.
• an apparatus for adaptive FEC includes an FEC encoder and an adaptive FEC device.
• the FEC encoder is for encoding k packets of source data into n packets, where n > k, and the n packets include redundant packets.
• the adaptive FEC device is for adaptively determining a number of the redundant packets to transmit with the encoded k packets, based at least one feedback message.
• the at least one feedback message may indicate the condition or status of the network over which the FEC encoded packets are to be transmitted.
• FIG. 1 shows a diagram for an exemplary encoding/decoding forward error correction (FEC) process to which the present invention may be applied
• FIG. 2 shows a diagram for a typical wireless video system architecture to which the present invention may be applied
• FIG. 3 shows a plot of received bit rate versus offered transmit rate for a WLAN link
• FIG. 4 shows a flow diagram for a method for adaptive FEC for video streaming over wireless networks in accordance with the principles of the present invention.
• the present invention is directed to adaptive forward error correction (FEC), in a preferred embodiment for video streaming over wireless networks.
• FEC forward error correction
• the present invention provides an adaptive FEC method and apparatus that improve video streaming performance by dynamically adjusting FEC strength based on network condition.
• the number of redundant packets that are transmitted over a wireless network is optimized based on network condition.
• a feedback signal is used to inform an adaptive FEC device at the sender side about network condition.
• the feedback signal can originate from, but is not limited to, the receiver, an intermediate node or the sender link layer.
• Monitoring network condition can be done at the sender side by accessing information from the sender wireless hardware such as retransmission attempts or by the use of Real-Time Control Protocol (RTCP) reports if RTP is used.
• RTCP Real-Time Control Protocol
• sender side information provides faster response to changes in network condition since RTCP reports are not sent frequently to conserve network resources. However, RTCP reports include more information and could be used to estimate available network bandwidth more accurately.
• a combination of feedback messages may also be used.
• the adaptive FEC device decides how many redundant packets to send. If the network condition is poor, a larger number of redundant packets are sent than in the case when the network condition is satisfactory. It is to be appreciated that while the present invention is described with respect to packet-level erasure FEC, other types of packet level FEC may also be employed in accordance with the present invention while maintaining the spirit and scope of the present invention.
• the instant description illustrates the principles of embodiments of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown in the embodiments described herein, embody the principles of the invention and are included within its spirit and scope.
• processor or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (“DSP”) hardware, read-only memory (“ROM”) for storing software, random access memory (“RAM”), and non-volatile storage.
• DSP digital signal processor
• ROM read-only memory
• RAM random access memory
• any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.
• any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements that performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function.
• the invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means that can provide those functionalities as equivalent to those shown herein.
• Packet-level erasure FEC is used to improve the reliability of wireless links. Erasure is easier to deal with since the exact location of the error is known.
• An (n, k) erasure FEC scheme encodes k source packets into n (n > k) packets. The encoding is such that any subset consisting of k packets is sufficient to reconstruct the source data.
• Source packets (k original packets) 110 are input to the encoder 120, and n packets 130 are output by the encoder 120.
• the n packets 130 include encoded data corresponding to the k packets 131 as well as redundant packets 132 (i.e., the shaded blocks in FIG. 1 ). Interleaving across a number of FEC packets may be used to improve error correction performance over a number of FEC blocks.
• the decoder 140 decodes the n packets (which includes the original k packets 110 and the redundant packets) to obtain reconstructed data 150 (which is the same as the original k packets 110).
• the packet loss rate of the wireless network fluctuates with time, network condition, network load, and so forth. Therefore, an adaptive forward error control scheme would be very useful to adjust the amount of redundancy transmitted over the channel.
• a feedback-based adaptive forward error correction method and apparatus are provided. If network conditions are satisfactory, there is no need to send a large number of redundant packets. In contrast, when the packet drop rate is high (above a predetermined threshold), then more redundant packets are transmitted.
• the sender side (which includes elements (205, 210, 215, 220, 225, and 230) is the FEC protected video source, which may be stored or live content.
• the wireless video system architecture 200 includes a video store 205 having an output connected in signal communication with an input of an FEC encoder 210.
• a video capture unit 215 having an output connected in signal communication with an input of a video encoder 220 that, in turn, includes an output connected in signal communication with the input of the FEC encoder 210, may be used in place of the video store 205.
• An output of the FEC encoder 210 is connected in signal communication with a first input of a buffer 225.
• An output of the buffer 225 is connected in signal communication with an input of a wireless network interface 230.
• a first output of the wireless network interface 230 is connected in signal communication with a wireless link (also referred to herein as a "wireless network") 235.
• a first and a second input of an adaptive FEC device 240 are respectively connected in signal communication with a second output of the wireless network interface 230 and with the wireless link 235.
• An output of the FEC device 240 is connected in signal communication with a second input of the buffer 225.
• a receiver 245 is connected in signal communication with the wireless link 235.
• the sender side FEC encoder 210 encodes the packets. Each packet includes an integral number of macroblocks and has a fixed number of bytes. The sender side then packs the video data in Real-Time Protocol/User Datagram Protocol (RTP/UDP) before being passed onto the wireless network 235 via the wireless network interface 230. The sender side transmits all the original packets all the time.
• the Adaptive FEC block decides the number of redundant packets to send based on one or more feedback messages (hereinafter "feedback message"). The feedback message could originate from the receiver side and is transmitted to the sender over RTCP or can be obtained from sender wireless retransmission information.
• the wireless retransmission information measures the number of times that a packet is retransmitted and could be used as an indicator of network condition.
• An effective adaptive approach could use both information with RTCP to provide more of a long term view of network condition while sender side information could be used to generate fast adaptation to network conditions.
• the Adaptive FEC device 240 decides the number of packets to send based on the feedback message.
• Many alternative techniques can be used to determine the quantity of packets to send. For example, one embodiment of the present invention makes use of an increase/decrease function.
• a "satisfactory" feedback message reduces the redundant packets to send by 1 to 0; otherwise, the number of redundant packets is increased by 1 up to n-k packets. To have a good adaptation rate, a large value of n- k should be used.
• an adjustment may be made using the RTCP receiver report.
• the fraction lost and the cumulative number of packets lost RTCP sender report fields can be used for adjustments. These fields represent the number of packets lost since the last receiver report and the total number of lost packets since the beginning of transmission, respectively.
• FIG. 3 wherein a plot of received bit rate versus offered transmit rate for a WLAN link is indicated generally by the reference numeral 300.
• a video transmitter (included in the wireless network interface 230) is sending data at a rate where the received rate increases as the offered transmit rate increases, then the FEC will improve the amount of information actually received. However, if the video transmitter is trying to send more data when it is operating on the portion of the curve that is flat, or worse yet decreasing, then the FEC will not help and may actually cause less video information to be received. For this reason, it is important for the video transmitter to have knowledge about what portion of the curve of FIG. 3 it is actually operating in. If it is operating in the decreasing portion of the curve, then it should not increase the amount of FEC data and should try to improve performance in some other way (e.g., reducing the bit rate) so that the operation goes to the increasing portion of the curve.
• some other way e.g., reducing the bit rate
• the video transmitter can add additional FEC information with good results.
• One way for the video transmitter to know where it is at on the curve is to use feedback (e.g., an RTCP report) from the receiver 245 or the wireless sender retransmission information. If the video receiver 245 receives a lower packet rate once the FEC is added, then the transmitter is operating on the decreasing portion of the curve. If the receiver 245 receives a higher packet rate once the FEC is added, then the transmitter is operating on the increasing portion of the curve. This information would already be available to the video transmitter if it were already receiving information on the dropped packets through some feedback path. Alternatively, the Media Access Control (MAC)-level retransmission information from the wireless transmitter could be used to estimate the number of dropped packets as already described.
• MAC Media Access Control
• FIG. 4 a method for adaptive FEC for video streaming over wireless networks is indicated generally by the reference numeral 400.
• a function block 405 encodes a video stream to generate fixed size video packets (e.g., MPEG2 transport packets), and passes control to a function block 410.
• the number of redundant packets to send (hereinafter also represented by the variable "X") is equal to zero (0).
• the function block 410 encodes every k source packet using erasure code FEC to generate n packets with n-k redundant packets, and then passes control to a function block 415.
• the function block 415 sends the first k source packets, and passes control to a decision block 420.
• the decision block 420 measures and determines the network condition. If the network condition is good (i.e., above a predetermined threshold), then control passes to a function block 425. Otherwise, if the network condition is bad (i.e., below the pre-determined threshold), then control passes to a decision block 430.
• the decision block 430 determines the operating region of the video transmitter with respect to received bit rate versus offered transmit rate (the curve shown in FIG. 3). In particular, the decision block 430 determines whether the transmitter is operating in an increasing region of the curve of FIG. 3 or in a non- increasing region of the curve. If the transmitter is operating in a non-increasing region of the curve, then control passes to a decision block 440 that sends X redundant packets, and passes control back to function block 410.
• control passes to a function block 450 that sends X min ⁇ X+1 , n-k) redundant packets, and passes control back to the function block 410. That is, the function block 450 sends one more redundant packet than the number previously sent, up to an upper limit of (n-k) redundant packets.
• the returning to function block 410 by function blocks 425, 440, and 450 enables adaptation of the number of redundant packets sent based on the network condition.
• teachings of the present invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or combinations thereof. Most preferably, the teachings of the present invention are implemented as a combination of hardware and software.
• the software is preferably implemented as an application program tangibly embodied on a program storage unit.
• the application program may be uploaded to, and executed by, a machine comprising any suitable architecture.
• the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPU"), a random access memory (“RAM”), and input/output (“I/O") interfaces.
• CPU central processing units
• RAM random access memory
• I/O input/output
• the computer platform may also include an operating system and microinstruction code.
• various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU.
• various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit.

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• Engineering & Computer Science (AREA)
• Quality & Reliability (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Detection And Prevention Of Errors In Transmission (AREA)
• Compression Or Coding Systems Of Tv Signals (AREA)
• Mobile Radio Communication Systems (AREA)

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Cited By (41)

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• 2005
  • 2005-06-14 EP EP05760965A patent/EP1817859A1/en not_active Ceased
  • 2005-06-14 US US11/792,184 patent/US8015474B2/en not_active Expired - Fee Related
  • 2005-06-14 BR BRPI0516632-2A patent/BRPI0516632A/pt not_active Application Discontinuation
  • 2005-06-14 WO PCT/US2005/020928 patent/WO2006060036A1/en active Application Filing
  • 2005-06-14 JP JP2007544325A patent/JP5425397B2/ja not_active Expired - Fee Related
  • 2005-06-14 CN CN200580039836.1A patent/CN101061659B/zh not_active Expired - Fee Related

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